Some of you may have noticed that when you install Virtual PC 2007 on a 64-bit host operating system it installs to the 32-bit program files directory. The reason why this happens is because, unlike Virtual Server, Virtual PC 2007 is a 32-bit application which uses 64-bit drivers when necessary.
In order to run on a 64-bit host operating system the drivers must be 64-bit themselves. But the only benefits of making the application 64-bit is the ability to access larger amounts of memory, and to run more virtual machines at the same time. While both of these things are very important for Virtual Server, they are not as important for Virtual PC.